Use the updated timestamp in the pull-request list
    
    Using the updated_on timestamp instead of the last_updated we are
    able to give the users a better idea of when exactly the PR got
    updated.
    The last_updated field is updated when any other PR is merged since
    then the merge_status cached value is reset.
    
    Fixes https://pagure.io/pagure/issue/2725
    
    Signed-off-by: Pierre-Yves Chibon <pingou@pingoured.fr>